Natural killer (NK) cells are a distinct population of lymphocytes with cytotoxic and immunoregulatory functions. There is now abundant evidence that NK killing is regulated by MHC class I molecules. The regulatory function of NK cells is mediated by the secretion of cytokines. The five components of this project will explore various aspects of NK cell biology. The first project focuses on the regulation of expression of inhibitory receptors belonging to the Ly49 and CD94/NKG2 families. In adult mice, these receptors are distributed to ensure self tolerance without compromising the ability of NK cells to detect alterations in self MHC molecules. Using fetal liver cell and bone marrow cell cultures, the mechanisms that regulate NK cell repertoire will be investigated. The second and third projects are devoted to the investigation of the activating NK cell receptor 2B4. This member of the Ig superfamily is expressed on both murine and human NK cells, and it recognizes the CD48 molecule. The second project addresses the functional aspects of human 2B4 and its in vivo relevance. The latter will be accomplished by analysis of immune function in 2B4 knock-out mice. In the third project, the molecular basis of signal transduction through the mouse 2B4 receptor will be studied. The two isoforms of mouse 2B4 that differ in their cytoplasmic region transmit inhibitory or activating signals. The proteins that associate with 2B4, including the newly discovered Spam-associated protein (SAP) will be characterized. The fourth project will address the cellular and molecular mechanisms that regulate the interaction of NK and B cells. Particularly, the role of IFN-gamma, IL-12, and novel 8IFN-gamma independent factors that facilitate NK-mediated switching of Ig isotypes will be determined. In the fifth Project, the information gathered on the NK cell receptors, and NK cell interactions with other cells will be applied to in vivo models of bone marrow transplantation. The basis of NK cell tolerance and its application towards the success of bone marrow allografts will be investigated. Although each of these five projects is independent, they all have the same central focus, are highly interrelated, and can benefit from a program core.